1. Field of the Invention
The invention relates to a low thermal resistance insulating support, which can more particularly be used for a power component such as a diode or a high frequency transistor. The bases or boxes incorporating such a support form part of the invention.
2. Description of the Prior Art
In an electronic component having a relatively high power dissipation (from a few Watts to a few hundred Watts or even higher) a large amount of heat is produced in a small volume. To remove the heat, a dissipative support (the base) is used and is made from a metal which is a good conductor of heat and electricity, which is itself in contact with air or a cooling device.
Moreover, at high frequency (from a few dozen to a few hundred megahertz or even higher) the connections of the component must be very large compared with the dimensions of the semiconducting pellet or pellets constituting the active part of the component. These connections are not then directly connected to the pellet or pellets and instead are welded to metal coatings deposited on an electrically insulating and thermally conductive intermediate support, which is indispensible for electrically insulating from at least part of the connections, while still permitting the removal of the heat.
Beryllium oxide supplies a dielectric material whose thermal conductivity (0.6 cal.s.sup.-1 cm.sup.-1) which is almost as good as that of gold (0.7 cal.s.sup.-1 cm.sup.-1) and whose linear expansion coefficient (7.7. 10.sup.-6) is matched to that of the silicon (7.6. 10.sup.-6), so that the pellet can be directly welded to the beryllium oxide metal coating.
However, a problem arises on increasing the power up to and beyond 100 Watts over an area of about 1 cm.sup.2. In order to improve heat removal, it has been attempted either to spread the semiconducting pellets over the metal conductive coating of the oxide so that the thermal cones intersect below the beryllium oxide, i.e. in the metal base or in cooling device, leading to large beryllium disks, or to reduce for an equivalent surface area the beryllium oxide thickness, which in the end leads to the same result.
In both cases, flaws occur in the beryllium oxide when using high temperature brazing with an alloy of silver and copper, which is the only reliable compatible alloy. On using low temperature brazing with so-called soft solders (alloy of lead, silver and indium) there are no BeO flaws, but the reliability is less satisfactory.